Electrical resistance element



p 14, 1965 w. 1.. GREENWOOD 3,206,702

ELECTRICAL RESISTANCE ELEMENT Filed July 1, 1965 INVENTOR.

WHITNEY GREENWOOD ATTORNEY United States Patent 3,206,702 ELECTRICAL RESISTANCE ELEMENT Whitney L. Greenwood, La Habra, Calif., assignor to Beckman Instruments, Inc., a corporation of California Filed July 1, 1963, Ser. No. 291,879 Claims. (Cl. 338-162) The present invention relates to electrical resistance elements and to methods of making such elements and is more particularly directed to resistance elements adapted for use in variable resistance devices and potentiometers which have a wiper adapted to traverse the resistance element.

The invention is primarly directed to resistance elements and devices using a resistance material which is applied as a layer or film on a non-conductive support or base. One such layer is formed of a mixture of glass and metal particles and commonly called cermet resistance material. Another resistance material suitable for such resistance devices is the well-known conductive plastic material, which is formed of a mixture of powdered plastic and conductive metal particles of carbon. A number of typical cermet resistance films and methods of making the same are described in US. Patents Nos. 2,950,995 and 2,950,996 issued in the name of Thomas M. Place, Sr., et al. andassigned to the same assignee as the present invention.

The resistance element and method of the present invention is primarily suited for rotary potentiometers and variable resistance devices which have full 360 or continuous rotation of the wiper or contact member. However, the invention is applicable to resistance elements deposited in any shape or form. It is normal practice, in 360 rotational devices, to provide an annular or are shaped layer of resistance material having a gap or open space between the ends of the resistance element. At the respective ends of the element on opposite sides of the gap are provided end terminals or electrical connections through which an electrical current is applied to the resistance element. The dead space or gap is usually filled with a suitable film of non-conductive material which permits passage of the wiper or movable I electrical contact thereover as it is moved from one end of the resistance material to the other. It has been found that, as the wiper passes from the resistance element proper to the non-conductive material in the dead space or gap during continuous rotational operation, an arc sometimes results due to the high current carried by the wiper. The arcing tends to erode the resistance film and to shorten the service life of the element.

When conductive and non-conductive materials, forming respectively the layer of conductive resistance material and the non-conductive bridge film are deposited onto a base and then fused to form the resistance element, there is a tendency for the conductive layer and nonconductive film to diffuse during the firing operation. This diffusion creates a small area of undetermined electrical conductivity adjacent the junction line of these materials which, when traversed by a wiper drawing current, is vulnerable to arcing. In such cases where an arc occurs, it will reoccur at the same point with each pass of the wiper. The present invention is directed to an arrangement whereby this diffusion of the two adjacent materials is eliminated thereby providing an element having much less tendency to are over during the transition of the wiper from the resistance material to the nonconductive bridge.

Accordingly, it is an object of the present invention to provide an improved bridge arrangement within a dead space or gap formed in a resistance element which ar- .rangement is less vulnerable to arcing when traversed by an electrical current carrying a wiper.

3,206,702 Patented Sept. 14, 1965 It is a more specific object of the present invention to provide an improved variable resistance device including a resistance element having a bridge section therein formed of a non-conductive film in which the resistance material, forming the element, and the non-conductive film are completely separate with no difiusion of the respective materials.

A further object of the present invention is to provide a method of forming a resistance element having a bridge section therein filled with a non-conductive film wherein the diffusion between the resistance layer and the non-conductive bridge film is substantially eliminated when the element is formed.

Further objects and advantages of the invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming a part of the specification.

In carrying .out the objects of the present invention, there is provided a resistance element in the form of a non-conductive base member having a flat surface area thereon upon which is deposited a resistance layer having a gap therein. The gap of the resistance element is substantially filled with a non-conductive film of material forming a bridge section across the gap. In order to reduce the electrical arcing in this region as a current carrying wiper traverses from the resistance layer to the bridge section and vice versa, a narrow channel or fissure is provided between the non-conductive bridge film and the resistance layer thereby sharply delineating the cut-off point between the resistance layer and the non-conductive bridge film.

As a further aspect of the present invention, there is provided a method of forming a resistance element in which a resistance layer is deposited on the surface of a non-conductive base member leaving a gap therein; a layer of material is then deposited in the gap substantially filling the gap but leaving narrow channels or fissures between the resistance layer and the non-conductive layer adapted to prevent difiusion of the respective layers during the firing thereof to a temperature calculated to fuse the respective materials.

For a better understanding of these and further aspects of the invention, reference may be had to the accompanying drawing in which:

FIG. 1 is a perspective view of a resistance element embodying the present invention;

FIG. 2 is an enlarged partial top view of an element showing the arrangement of the bridge section between the terminal ends of the resistance layer of such a resistance element; and

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 2 further illustrating the bridge section of the resistance element, and showing a conductive wiper traversing from the resistance layer onto the bridge section.

Referring now to FIG. 1 of the drawing, there is shown a resistance element of the type adapted for use in the variable resistance art. The resistance element comprising a substrate base member 2 of dielectric material which may be in the form of a circular disc or wafer. The member is usually'formed of an unglazed homogeneous, non-porous ceramic material, such as steatite or alumina or other non-conductive material well-known in the art. While the illustrated resistance device is in the form of a circular disc of the type used in rotary-turn potentiometers or rotary variable resistors, it will be understood that the present invention is not limited to resistance devices of this particular shape and that the invention is equally applicable to the manufacture of resistance devices of rectangular shape or any other suitable configuration.

In conventional practice, the substrate or base member 2 is molded, fired and then ground or lapped to provide a smooth relatively flat surface 2a for supporting a resistance film or layer 3 thereon. The resistance layer 3 forms a conductive path for an electrical current which is applied to the resistance layer through terminal members 4 and 5. In the illustrated embodiment of the invention, the terminal members or conductors 4 and 5 are in the form of metal ribbons or leads imbedded in the base memher 2. These terminal conductors may also be in the form of a metal film deposited on the non-conductive base member. The noble metals, because of their resistance to oxidation, have been found best suited for use as terminal conductors and various alloys of gold, silver, platinum, palladium or rhodium are available with a coefficient of expansion that matches, to some extent, that of the ceramic base member 2.

The resistance element or layer 3 may be formed of a suitable conductive or resistance material such as conductive plastic material, having a carbonaceous metal dispersed throughout the fused plastic or of a cermet resistance material comprising a non-conductive glass binder material having minute particles of noble metal or metal alloy dispersed throughout the element and having trace amounts of other material added When desired. Particular examples of cermet materials and their method of manufacture are fully explained in the aforementioned Place et al. Patent Nos. 2,950,995 and 2,950,996 assigned to the assignee of the present invention. In cermet materials, the mixture of glass and metal particles, after fusion at a temperature below that of the metal constituent, forms a continuous element having a hard, smooth, glassy surface. A glass-metal cermet mixture is predominantly glass with relatively small percentages of metal therein, depending upon the particular value of resistance desired.

The resistance material is applied to the base member 2 in a configuration desired for the particular application of the resistance device. For example, in the disclosed disc-type element, the film or layer is annular or areshaped and may be applied to the base member 2 by any suitable operation well known in the art such as brushing, spraying, stenciling or silk screening. Those areas on the surface of the substrate or non-conductive base member, upon which the resistance material is not desired, are masked in a manner well known in the art to prevent the deposition of the coating thereon. A space or terminal gap on the base member, between the terminal members 4 and 5, is suitably masked so that, when the resistance material is applied, the gap is then left vacant.

Within the space is deposited a bridging film or layer 6 of non-conductive material, such as glass or plastic having no conductive particles therein. This non-conductive layer is of substantially the same thickness as that of the resistance material in order to provide a smooth surface so that the Wiper of a potentiometer or variable resistor may easily traverse over this bridge section 6 from one end of the resistance layer to the other. The glass, or plastic material in the bridge area should be similar to that used in the formation of the resistance mixture so that, upon fusing of the material the materials in the bridge area fuse at the same or similar temperatures as the resistance mixture. Thus, the non-conductive film or layer 6 is preferably formed of glass when the resistance material is cermet and may be of plastic when the resistance material is conductive plastic.

Referring now to FIGS. 2 and 3, it will be noted that the film of non-conductive material 6 in the gap does not completely fill the gap flush to the ends of the resistance material 3. That is, the non-conductive bridging layer 6 is screened or otherwise deposited into the space or gap between the respective end portions of the conductive resistance film, partially filling the gap but leaving thin narrow channels or fissures 7 between the non-conductive bridging layer and each end of the conductive resistance layer 3. By controlling the screened widths of the fissures 7, the width of the fissures can be closely maintained so that no diffusion results between the respective materials during firing.

After the resistance layer 3 and the non-conducting bridge layer 6 have been deposited on the surface 2a of the base member 2, with suitable fissures 7 separating the respective adjoining portions of these layers, the base and layers are preferably permitted to dry in circulating warm air for a short period. The masking (if necessary) is removed from the uncoated surface of the unit and the unit is fired in a kiln or furnace which may be of any conventional type for heating materials to a high temperature. It is preferable to fire the resistance element in a clean atmosphere and to raise the temperature of the element to the fusion temperature without addition of extraneous gases to the atmosphere.

As illustrated in FIG. 3, the adjacent portions of the respective layers are maintained separate during the fusion operation and, while the layers may actually soften and move into substantially complete abutment, there is no diffusion of one material into the other. Consequently, none of the metal particles from the resistance material 3 diffuse into the non-conductive layer of cermet or plastic and a sharp break-off line of conductance is maintained across the resistance element.

As may be seen in FIG. 3, a wiper or electrical contact member, such as the arm and contact assembly 8, when traversing the surface of the resistance element, is de signed to pass over the fissure between the non-conductive film 6 and the resistance layer 3. For example, the contact element 8a of the assembly is formed with an annular shape having a width or dimension in direction of rotation of sufficient length to permit smooth transition from the resistance material 3 to the bridging film section 6. As the wiper moves from the resistance material 3 onto the non-conductive bridge section 6, there is little or no arcing because there are no metal particles from the resistance material displaced into the bridge section. The result is an abrupt or definite line of conductive termination between the resistance material and the nonconductive bridge section.

While in accordance with the patent statutes there has been provided what at present is considered to be the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, the aim of the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A resistance element comprising:

a non-conductive base member having a substantially smooth surface area thereon;

a layer of resistance material deposited on said flat surface of said base member, said layer of resistance material having a gap therein; and

a non-conductive film disposed in said gap, said film being of substantially the same thickness as said layer and so arranged as to provide narrow fissures on either side thereof between said non-conductive film and said resistance layer.

2. A resistance element comprising:

a non-conductive base member having a substantially smooth surface area thereon;

a layer of resistance material deposited on said fiat surface of said base member, said layer of resistance material having a gap therein;

conductive terminal members electrically connected to said layer of resistance material adjacent said gap therein; and

a non-conductive film disposed in said gap, said film being of substantially the same thickness as said layer and so arranged as to provide narrow fissures on either side thereof between said non-conductive film and said resistance layer.

3. A resistance element comprising:

a non-conductive base member having a substantially smooth surface area thereon;

a pair of conductive terminal members afiixed to said base member and having a space therebetween;

a layer of resistance material aflixed on said smooth surface of said base member and overlying portions of said terminal members to provide an electrical connection therewith, said layer of resistance material having a gap therein within said space between said terminal members; and

a non-conductive film disposed in said gap, said film being of substantially the same thickness as said layer and so constructed and arranged to provide narrow fissures on either side thereof between said nonconductive film and said resistance layer.

4. A resistance element comprising:

a non-conductive base member having a substantially smooth surface thereon; I

a pair of conductive terminal members afiixed to said base member and having a space therebetween;

a circuituous layer of resistance material atfixed on said smooth surface of said base member and overlying portions of said terminal members to provide electrical connection therewith, said circuituous layer of resistance material having a gap therein within said space between said terminal members; and

a non-conductive film disposed in said gap, said film being of substantially the same thickness as said layer and so arranged as to provide narrow fissures on either side thereof between said non-conductive film and said resistance layer.

5. A variable resistance device comprising:

a non-conductive base member having a substantially smooth surface area thereon;

a layer of resistance material deposited on said flat surface of said base member, said layer of resistance material having a gap therein;

a non-conductive bridging film disposed in said gap, said film being of substantially the same thickness as said layer and so constructed and arranged to provide narrow fissures on either side thereof between said non-conductive film and said resistance layer; and

an electrically conductive Wiper mounted above said element and adapted to traverse the surfaces of said resistance layer and said non-conductive film, said wiper having a contacting surface of a width sulficient to bridge the width of said fissures when traversing from said layer to said film and vice versa.

6. A variable resistance device comprising:

a non-conductive base member having a substantially smooth surface area thereon;

a pair of conductive terminal members affixed to said base member and having a space therebetween;

a layer of resistance material affixed on said smooth surface of said base member and overlying portions of said terminal members to provide an electrical connection therewith, said layer of resistance material having a terminal gap therein within said space between said terminal members;

a non-conductive film disposed in said terminal gap, said film being of substantially the same thickness as said layer and so constructed and arranged to provide narrow fissures on either side thereof between said non-conductive film and said resistance layer; and

a movable electrical wiper mounted above said layer of resistance material and adapted to traverse the surface of said resistance layer and said non-conductive film, said wiper having an annular contacting surface of sufiicient width to bridge the width of said fissures when traversing from said layer to said film and vice verse.

7. A variable resistance device comprising:

a non-conductive base member having a substantially smooth surface area thereon;

a pair of conductive terminal members aflixed to said base member and having a space therebetween;

a circuituous layer of resistance material aflixed to said smooth surface of said base member and overlying portions of said terminal members to provide electrical connection therewith, said circuituous layer of resistance material having a terminal gap therein within said space between said terminal members;

a non-conductive bridging film disposed in said terminal gap, said film being of substantially the same thickness as said resistance layer and so constructed and arranged to provide narrow fissures on either side thereof between said bridging film and the terminal ends of said resistance layer; and

a rotatable wiper mounted above said resistance layer adapted for continuous rotation to traverse the circuituous surface of said resistance layer and said bridging film said contact member having an annular contact of sufiicient width to extend across the width of said fissures when traversing from said layer to said film and vice versa.

8. A method of making a resistance element comprising the steps of:

providing a non-conductive substrate member having a substantially fiat surface area thereon;

applying to said flat surface of said substrate member a layer of resistance material in the form of a mixture of finely divided electrically conductive met-a1 particles and a fusible bonding material in a configuration having a gap therein, said fusible bonding material having a lower melting point than said metal particles;

applying a layer of fusible bonding material into said gap leaving narrow fissures between the respective adjacent portions of said resistance material and said fusible bonding material, said layer of fusible bonding material being deposited on said substrate in said gap at substantially the same thickness as said layer of resistance material; and

firing said member to a temperature lower than the melting point of said metal particles and exceeding the fusion temperature of said bonding material thereby to provide a thin resistance layer having a non-conductive gap therein.

9. A method of making a resistance element comprising the step of:

providing a non-conductive substrate member having a substantially flat surface area thereon;

applying a pair of terminal strips to said substrate member separated by a short space;

applying a film of cermet resistance material of finely divided glass and conductive metal particles on said surface of said substrate base member and overlying said exposed surface of said conductor strip in a configuration having a gap therein disposed in the region between said terminal strips, said finely divided glass having a lower melting point than said conductive metal particles;

depositing a layer of finely divided non-conductive fusible glass bonding material into said gap leaving narrow fissures between the respective adjacent portions of said cermet material and said fusible glass material, said layer of finely divided non-conductive fusible glass bonding material being deposited on said substrate in said gap at substantially the same thickness as said layer of resistance material; and

firing said member to a temperature lower than the melting point of said metal particles and exceeding the fusion temperature of said glass bonding material thereby to provide a thin resistance layer having a non-conductive gap therein.

10. A method of making a resistance element comprising the steps of:

ber a layer of conductive plastic material comprising carbon particles and plastic bonding material in a finely divided mixture, said layer being deposited in a configuration having a gap therein, said plastic bonding material having a lower melting point than said carbon particles;

applying a bridging layer of said plastic bonding maraising the temperature of said member to a temperature lower than the melting point of said carbon particles and exceeding the fusion temperature of said plastic material thereby to provide a thin resistance layer having a non-conductive gap therein.

References Cited by the Examiner UNITED STATES PATENTS 2,082,980 6/37 Schellenger 338-l62 X 2,704,316 3/55 DeBell 338--l74 X 2,906,984 9/59 Mucher et al. 338171 3,044,901 7/62 Garnsworthy l172l2 3,058,842 10/ 62 Kahan et al. 117-l07 FOREIGN PATENTS 1,103,756 6/55 France.

RICHARD M. WOOD, Primary Examiner. 

1. A RESISTANCE ELEMENT COMPRISING A NON-CONDUCTIVE BASE MEMBER HAVING A SUBSTANTIALLY SMOOTH SURFACE AREA THEREON; A LAYER OF RESISTANCE MATERIAL DEPOSITED ON SAID FLAT SURFACE OF SAID BASE MEMBER, SAID LAYER OF RESISTANCE MATERIAL HAVING A GAP THEREIN; AND A NON-CONDUCTIVE FILM DISPOSED IN SAID GAP, SAID FILM BEING OF SUBSTANTIALLY THE SAME THICKNESS AS SAID LAYER AND SO ARRANGED AS TO PROVIDE NARROW FISSURES ON EITHER SIDE THEREOF BETWEEN SAID NON-CONDUCTIVE FILM AND SAID RESISTANCE LAYER. 